**H3PO4 is less acidic than H3PO3: Explanation**

**Introduction**
H3PO4 and H3PO3 are both acids belonging to the phosphoric acid family. However, H3PO4 is known as phosphoric acid, while H3PO3 is known as phosphorous acid. Despite their similarities, H3PO4 is less acidic than H3PO3 due to differences in their molecular structures and ionization tendencies.

**1. Structure of H3PO4**
- H3PO4 consists of one central phosphorus atom bonded to four oxygen atoms and three hydrogens.
- The four oxygen atoms surround the central phosphorus atom in a tetrahedral arrangement.
- Each oxygen atom in H3PO4 can donate a proton (H+), resulting in the formation of four acidic hydrogen ions.

**2. Structure of H3PO3**
- H3PO3 consists of one central phosphorus atom bonded to three oxygen atoms and three hydrogens.
- The three oxygen atoms surround the central phosphorus atom in a trigonal planar arrangement.
- Unlike H3PO4, only two of the oxygen atoms in H3PO3 can donate a proton.
- The third oxygen atom in H3PO3 is bonded to phosphorus via a double bond, making it less likely to donate a proton.

**3. Ionization of H3PO4 and H3PO3**
- The ionization of an acid occurs when it donates a proton (H+) in an aqueous solution.
- H3PO4, being a stronger acid, readily ionizes in water to release all four of its acidic hydrogen ions.
- On the other hand, H3PO3, being a weaker acid, ionizes partially in water, releasing only two of its acidic hydrogen ions.
- The presence of the double bond in H3PO3 hinders the ionization of the third hydrogen atom, reducing its overall acidity.

**4. Acid Strength**
- The strength of an acid is determined by its ability to donate protons.
- Since H3PO4 can donate more protons than H3PO3, it is considered a stronger acid.
- Therefore, H3PO4 is less acidic than H3PO3 because it ionizes more completely in water and releases more hydrogen ions.

In conclusion, H3PO4 is less acidic than H3PO3 due to differences in their molecular structures and ionization tendencies. H3PO4's tetrahedral structure allows for the donation of all four acidic hydrogen ions, while H3PO3's trigonal planar structure and double bond hinder the ionization of the third hydrogen atom. As a result, H3PO4 is considered a stronger acid and exhibits greater acidity compared to H3PO3.

First draw the Lewis structures of each acid. You will see that for H3PO3 one of the hydrogens is bonded directly to phosphorus. This is not an acidic hydrogen and therefore H3PO3 is diprotic. Another way of thinking about this is to consider the polarization of each acid. H3PO3 is more polar, so the O-H bonds will be weaker, making the H+ easier to come off, and causing H3PO3 to be more acidic than H3PO4.